.TH std::format 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::format \- std::format

.SH Synopsis
   Defined in header <format>
   template< class... Args >
   std::string format( std::format_string<Args...> fmt, Args&&...     \fB(1)\fP \fI(since C++20)\fP
   args );
   template< class... Args >
   std::wstring format( std::wformat_string<Args...> fmt, Args&&...   \fB(2)\fP \fI(since C++20)\fP
   args );
   template< class... Args >

   std::string format( const std::locale& loc,                        \fB(3)\fP \fI(since C++20)\fP

                       std::format_string<Args...> fmt, Args&&...
   args );
   template< class... Args >

   std::wstring format( const std::locale& loc,                       \fB(4)\fP \fI(since C++20)\fP

                        std::wformat_string<Args...> fmt, Args&&...
   args );

   Format args according to the format string fmt, and return the result as a string.
   If present, loc is used for locale-specific formatting.

   1) Equivalent to return std::vformat(fmt.get(), std::make_format_args(args...));.
   2) Equivalent to return std::vformat(fmt.get(), std::make_wformat_args(args...));.
   3) Equivalent to return std::vformat(loc, fmt.get(),
   std::make_format_args(args...));.
   4) Equivalent to return std::vformat(loc, fmt.get(),
   std::make_wformat_args(args...));.

   Since P2216R3, std::format does a compile-time check on the format string (via the
   helper type std::format_string or std::wformat_string). If it is found to be invalid
   for the types of the arguments to be formatted, a compilation error will be emitted.
   If the format string cannot be a compile-time constant, or the compile-time check
   needs to be avoided, use std::vformat
   or std::runtime_format on fmt
   (since C++26) instead.

   The following requirements apply to each type T in Args, where CharT is char for
   overloads (1,3), wchar_t for overloads (2,4):

     * std::formatter<T, CharT> must satisfy BasicFormatter
     * std::formatter<T, CharT>::parse() must be constexpr since P2216R3 (std::vformat
       does not have this requirement)

.SH Parameters


             an object that represents the format string. The format string consists of
               * ordinary characters (except { and }), which are copied unchanged to
                 the output,
               * escape sequences {{ and }}, which are replaced with { and }
                 respectively in the output, and
               * replacement fields.

             Each replacement field has the following format:

             { arg-id (optional) }               \fB(1)\fP
             { arg-id (optional) : format-spec } \fB(2)\fP

             1) replacement field without a format specification
             2) replacement field with a format specification

                           specifies the index of the argument in args whose value is
                           to be used for formatting; if it is omitted, the arguments
                           are used in order.
             arg-id      -
                           The arg-id s in a format string must all be present or all
                           be omitted. Mixing manual and automatic indexing is an
                           error.
   fmt     -               the format specification defined by the std::formatter
             format-spec - specialization for the corresponding argument. Cannot start
                           with }.

               * For basic types and standard string types, the format specification is
                 interpreted as standard format specification.
               * For chrono types, the format specification is interpreted as chrono
                 format specification.

               * For range types, the format specification is interpreted
                 as range format specification.
               * For std::pair and std::tuple, the format specification
                 is interpreted as tuple format specification.
               * For std::thread::id and std::stacktrace_entry, see       (since C++23)
                 thread id format specification and stacktrace entry
                 format specification.
               * For std::basic_stacktrace, no format specifier is
                 allowed.

               * For std::filesystem::path, see path format               (since C++26)
                 specification.

               * For other formattable types, the format specification is determined by
                 user-defined formatter specializations.
   args... - arguments to be formatted
   loc     - std::locale used for locale-specific formatting

.SH Return value

   A string object holding the formatted result.

.SH Exceptions

   Throws std::bad_alloc on allocation failure. Also propagates exception thrown by any
   formatter.

.SH Notes

   It is not an error to provide more arguments than the format string requires:

 std::format("{} {}!", "Hello", "world", "something"); // OK, produces "Hello world!"

   As of P2216R3, it is an error if the format string is not a constant expression.
   std::vformat can be used in this case.

 std::string f(std::string_view runtime_format_string)
 {
     // return std::format(runtime_format_string, "foo", "bar"); // error
     return std::vformat(runtime_format_string, std::make_format_args("foo", "bar")); // OK
 }

  std::runtime_format can be used directly on std::format instead of std::vformat
  which requires std::basic_format_args as an argument.
                                                                                    (since
  std::string f(std::string_view runtime_format_string)                             C++26)
  {
      return std::format(std::runtime_format(runtime_format_string), "foo", "bar");
  }

.SH Example


// Run this code

 #include <format>
 #include <iostream>
 #include <string>
 #include <string_view>

 template<typename... Args>
 std::string dyna_print(std::string_view rt_fmt_str, Args&&... args)
 {
     return std::vformat(rt_fmt_str, std::make_format_args(args...));
 }

 int main()
 {
     std::cout << std::format("Hello {}!\\n", "world");

     std::string fmt;
     for (int i{}; i != 3; ++i)
     {
         fmt += "{} "; // constructs the formatting string
         std::cout << fmt << " : ";
         std::cout << dyna_print(fmt, "alpha", 'Z', 3.14, "unused");
         std::cout << '\\n';
     }
 }

.SH Output:

 Hello world!
 {}  : alpha
 {} {}  : alpha Z
 {} {} {}  : alpha Z 3.14

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

     DR    Applied to      Behavior as published               Correct behavior
   P2216R3 C++20      throws std::format_error for     invalid format string results in
                      invalid format string            compile-time error
                      objects that are neither
   P2418R2 C++20      const-usable nor copyable        allow formatting these objects
                      (such as generator-like objects)
                      are not formattable
   P2508R1 C++20      there's no user-visible name for the name basic_format_string is
                      this facility                    exposed

.SH See also

   format_to   writes out formatted representation of its arguments through an output
   (C++20)     iterator
               \fI(function template)\fP
   format_to_n writes out formatted representation of its arguments through an output
   (C++20)     iterator, not exceeding specified size
               \fI(function template)\fP
